1. Field of the Invention
The present invention relates to a polymer gel capable of a phase transition occurring due to a stimulus, such as heat, light or an electric field, a method of manufacturing such a polymer gel, and an actuator which employs such a polymer gel.
2. Related Background Art
Recently, attention has been paid to the phenomenon of polymer gels which experience a change in their volume upon being subjected to various types of stimuli. Polymer gels exhibiting this phenomenon include polyacrylamide and polyvinyl alcohol.
Conventionally, such polymer gels are formed in a bar-like or string-like bulky shape. This is advantageous because manufacture is easily facilitated and because the resulting products are visually recognizable. However, since the phase transition occurs causing a change in the volume due to a diffusion of water contained in the system, formation of a polymer gel in a bulky shape is not desirable because it hinders the sensitivity of the polymer gel's phase transition and decreases the speed thereof.
Spherically shaped polymer gels are also known, such polymer gels being described on pages 301 to 305 in "High Polymer, Japan" by Mori, Vol. 13, No. 145 and on pages 40 to 47 in "Journal of the Japan Society of Mechanical Engineers" by Tatara, Vol. 76, No. 656. However, these polymer gels exhibit non-uniform particle size and composition.
It is apparent that particulate polymer gels having a large surface area are desirable from the viewpoint of the smoothness with which a substance diffuses from the polymer gel to result in a change in the volume thereof.
In the case of a nonaqueous monomer such as styrene, pearl polymerization is generally employed to form a particulate polymer. When a water-soluble monomer is polymerized to form a particulate polymer, the monomer is emulsified in a nonaqueous medium to form reversed phase micelles. However, polymerization of the water-soluble monomer has difficulties, for example, in the selection of a surface active agent which is used to form the reversed phase micelles.
In particular, polymerization of acrylamides or the like, the polymer of which exhibits a change in volume requires crosslinkage of the polymer by a low molecular weight crosslinking agent, thus necessitating the requirement that the particles have a uniform composition. A non-uniform composition of the polymer leads to non-uniform changes in the volume thereof, which causes degradation in the sharpness of the phase transition and reduction in the response speed.
Needless to say, when such a polymer gel is used to form, for example, an actuator, the resultant actuator will not respond quickly to a stimulus and hence will not ensure precise operation.
A particulate polymer gel may also be obtained by pulverizing the polymer gel obtained by solid phase polymerization. However, the particles obtained by pulverizing the solid phase polymerized crystals or bulk polymer gels are non-uniform in their composition and size, and this makes it impossible for an actuator made of such a particulate polymer gel to operate quickly and precisely.
For the above-described reasons, such an actuator made of this polymer gel is slower in responding to a stimulus, has a low torque, and is not efficient when compared with a mechanical actuator.